Detection apparatus and method for a flexible pipe

ABSTRACT

A detection apparatus and method arranged to detect defects within a flexible pipe at least partially surrounded by seawater. The detection apparatus comprises a seawater electrode, an impedance monitor and a processor. The seawater electrode is arranged to be in contact with seawater surrounding at least part of a flexible pipe. The impedance monitor is arranged to measure the impedance between a metallic structural component of the flexible pipe extending at least partially along the length of the flexible pipe and the seawater electrode in response to an electrical test signal applied to the seawater electrode. The processor is arranged to determine the distance from the seawater electrode to a pipe defect electrically connecting the metallic structural component to seawater using the measured impedance.

The present invention relates to a detection apparatus and method. Inparticular, the present invention relates to a detection apparatusarranged to detect defects within a flexible pipe, and a method ofdetecting defects within a flexible pipe. Particular embodiments relateto a detection apparatus suitable for detecting a pipe defect within anexisting flexible pipe installation.

Traditionally flexible pipe is utilised to transport production fluids,such as oil and/or gas and/or water, from one location to another.Flexible pipe is particularly useful in connecting a sub-sea location(which may be deep underwater, say 1000 m or more) to a sea levellocation. The pipe may have an internal diameter of typically up toaround 0.6 m. Flexible pipe is generally formed as an assembly of aflexible pipe body and one or more end fittings. The pipe body istypically formed as a combination of layered materials that form apressure-containing conduit. The pipe structure allows large deflectionswithout causing bending stresses that impair the pipe's functionalityover its lifetime. The pipe body is generally built up as a combinedstructure including metallic and polymer layers.

In many known flexible pipe designs the pipe body includes one or morepressure armour layers. The primary load on such layers is formed fromradial forces. Pressure armour layers often have a specific crosssection profile to interlock so as to be able to maintain and absorbradial forces resulting from outer or inner pressure on the pipe. Thecross sectional profile of the wound wires which thus prevent the pipefrom collapsing or bursting as a result of pressure are sometimes calledpressure-resistant profiles. When pressure armour layers are formed fromhelically wound wire forming hoop components, the radial forces fromouter or inner pressure on the pipe cause the hoop components to expandor contract, putting a tensile load on the wires.

In many known flexible pipe designs the pipe body includes one or moretensile armour layers. The primary loading on such a tensile armourlayer is tension. In high pressure applications, such as in deep andultra-deep water environments, the tensile armour layer experiences hightension loads from a combination of the internal pressure end cap loadand the self-supported weight of the flexible pipe. This can causefailure in the flexible pipe since such conditions are experienced overprolonged periods of time.

Unbonded flexible pipe has been used for deep water (less than 3,300feet (1,005.84 m)) and ultra-deep water (greater than 3,300 feet)developments. It is the increasing demand for oil which is causingexploration to occur at greater and greater depths where environmentalfactors are more extreme. For example, in such deep and ultra-deep waterenvironments, ocean floor temperature increases the risk of productionfluids cooling to a temperature that may lead to pipe blockage.Increased depths also increase the pressure associated with theenvironment in which the flexible pipe must operate. As a result theneed for high levels of performance from the layers of the flexible pipebody is increased. Flexible pipe may also be used for shallow waterapplications (for example less than around 500 m depth) or even forshore (overland) applications.

One way to improve the load response and thus performance of armourlayers is to manufacture the layers from thicker and stronger and thusmore robust materials. For example, for pressure armour layers in whichthe layers are often formed from wound wires with adjacent windings inthe layer interlocking, manufacturing the wires from thicker materialresults in the strength increasing appropriately. However, as morematerial is used, the weight of the flexible pipe increases. Ultimatelythe weight of the flexible pipe can become a limiting factor in usingflexible pipe. Additionally manufacturing flexible pipe using thickerand thicker material increases material costs appreciably, which is alsoa disadvantage.

Regardless of measures taken to improve the performance of armour layerswithin a pipe body, there remains a risk of defects arising within aflexible pipe. A defect may comprise damage to an outer wall of aflexible pipe body resulting in the ingress of seawater into an annuluswithin the pipe body such that seawater fills voids between the armourlayer wires and other structural elements of the pipe. Armour layerwires and other structural elements are typically manufactured fromsteel or other metallic materials, which are vulnerable to acceleratedcorrosion upon contact with seawater. If such a defect is not detectedpromptly then the structural integrity of the pipe body can becompromised. Detection of defects has previously often required visualinspection of the pipe body, which can be hazardous, particular for deepwater and ultra-deep water installations.

Certain embodiments of the invention provide the advantage that a defectwithin a pipe body can be detected without requiring periodic visualinspection. Defects can then be repaired, or the pipe body replaced.Detectable defects include a breach of the outer wall of a flexible pipeand the ingress of seawater into a pipe body annulus. Certainembodiments of the invention allow the location of a defect to bedetermined, with sufficient accuracy to allow a repair to be effected.Certain embodiments of the invention allow a defect to be located for anexisting pipe body installation. That is, the detection apparatus is notreliant on having been installed or coupled to the pipe body in advanceof a defect occurring and the detection apparatus can be used withexisting pipe body designs that are already widely deployed.

According to a first aspect of the present invention there is provided adetection apparatus arranged to detect defects within a flexible pipe atleast partially surrounded by seawater, the detection apparatuscomprising: a seawater electrode arranged to be in contact with seawatersurrounding at least part of a flexible pipe; an impedance monitorarranged to measure the impedance between a metallic structuralcomponent of the flexible pipe extending at least partially along thelength of the flexible pipe and the seawater electrode in response to anelectrical test signal applied to the seawater electrode; and aprocessor arranged to determine the distance from the seawater electrodeto a pipe defect electrically connecting the metallic structuralcomponent to seawater using the measured impedance.

The seawater electrode may be arranged to move relative to the flexiblepipe, the impedance monitor is arranged to measure the impedance betweenthe metallic structural component and the seawater electrode at two ormore positions and the processor is arranged to determine the distancefrom the seawater electrode to the pipe defect for each position and totriangulate the location of the pipe defect.

The seawater electrode may be arranged to be lowered through theseawater surrounding the flexible pipe or the seawater electrode iscoupled to a steering mechanism such that the location of the seawaterelectrode relative to the flexible pipe can be controlled.

The detection apparatus may further comprise two or more spaced apartseawater electrodes separately coupled to the impedance monitor whereinthe impedance monitor is arranged to measure the impedance between themetallic structural component and each seawater electrode and theprocessor is arranged to determine the distance from each seawaterelectrode to the pipe defect and to triangulate the location of the pipedefect.

The detection apparatus may further comprise a position locator coupledto the seawater electrode and arranged to provide an indication of thelocation of the seawater electrode relative to the flexible pipe;wherein the processor is arranged to determine the location of the pipedefect from the measured impedance and the position of the seawaterelectrode.

The seawater electrode may comprise a loop arranged to pass around theflexible pipe, the loop incorporating two or more conducting elementsspaced around the flexible pipe, the conducting elements being arrangedto be coupled together and connected to the impedance monitor orseparately connected to the impedance monitor.

The processor may be further arranged to determine the location of thepipe defect using a three dimensional model of the location of theflexible pipe.

The impedance monitor may be arranged to measure the impedance for testsignal applied to the seawater electrode at a first frequency selectedaccording to the distance between the seawater electrode and theflexible pipe.

The impedance monitor may be arranged to measure the impedance for twoor more test signal frequencies applied to the seawater electrode, andthe processor is arranged to determine the distance from the seawaterelectrode to a pipe defect by comparison of the measured impedances atthe first and second frequencies.

The impedance monitor may be arranged to apply electrical test signalsto the seawater electrode at a plurality of frequencies between 10 Hzand 100 kHz.

According to a second aspect of the present invention there is provideda method of detecting defects within a flexible pipe at least partiallysurrounded by seawater, the method comprising: immersing a seawaterelectrode into seawater surrounding at least part of a flexible pipe;coupling an impedance monitor between a metallic structural component ofthe flexible pipe extending at least partially along the length of theflexible pipe and the seawater electrode; generating an electrical testsignal and applying the electrical test signal to the seawaterelectrode; measuring the impedance between the metallic structuralcomponent and the seawater electrode in response to the test signal; anddetermining the distance from the seawater electrode to a pipe defectelectrically connecting the metallic structural component to seawaterusing the measured impedance.

The method may further comprise: moving the seawater electrode relativeto the flexible pipe; measuring the impedance between the metallicstructural component and the seawater electrode at two or morepositions; determining the distance from the seawater electrode to thepipe defect for each position; and triangulating the location of thepipe defect.

The method may further comprise: immersing two or more spaced apartseawater electrodes into seawater surrounding at least part of theflexible pipe, each seawater electrode being separately coupled to theimpedance monitor; measuring the impedance between the metallicstructural component and each seawater electrode; determining thedistance from each seawater electrode to the pipe defect; andtriangulating the location of the pipe defect.

According to a third aspect of the present invention there is provided adetection apparatus arranged to detect defects within a flexible pipe atleast partially surrounded by seawater, the detection apparatuscomprising: a seawater electrode in contact with seawater surrounding atleast part of a flexible pipe and arranged to generate an electric fieldwithin the seawater relative to the potential of a metallic structuralcomponent of the flexible pipe extending at least partially along thelength of the flexible pipe: and an electric field probe arranged tomeasure an electric field vector within the seawater surrounding theflexible pipe; wherein the measured electric field vector is indicativeof the direction from the electric field probe to a pipe defectelectrically connecting the metallic structural component to seawater.

The detection apparatus may further comprise: a position locator coupledto the electric field probe and arranged to provide an indication of thelocation of the electric field probe relative to the flexible pipe; anda processor arranged to determine the location of the pipe defect fromthe measured electric field vector and the position of the electricfield probe.

The seawater electrode may be arranged to generate an electric field ata first frequency, and the processor is arranged to synchronise themeasured electric field vector to the first frequency to determine thelocation of the pipe defect.

The electric field probe may be arranged to move relative to theflexible pipe and the processor is arranged to triangulate the locationof the pipe defect from electric field vectors measured at two or morepositions of the electric field probe.

The electric field probe may be arranged to be lowered through theseawater surrounding the flexible pipe or the electric field probe iscoupled to a steering mechanism such that the location of the seawaterelectrode relative to the flexible pipe can be controlled.

The processor may be further arranged to determine the location of thepipe defect using a three dimensional model of the location of theflexible pipe.

The detection apparatus may further comprise: an orientation sensorcoupled to the electric field probe and arranged to determine the threedimensional orientation of the electric field probe; wherein theelectric field probe is arranged to measure a three dimensional electricfield vector.

According to a fourth aspect of the present invention there is provideda method of detecting defects within a flexible pipe at least partiallysurrounded by seawater, the method comprising: immersing a seawaterelectrode into seawater surrounding at least part of a flexible pipe;generating an electric field using the seawater electrode within theseawater relative to the potential of a metallic structural component ofthe flexible pipe extending at least partially along the length of theflexible pipe; and measuring an electric field vector within theseawater surrounding the flexible pipe; wherein the measured electricfield vector is indicative of the direction from the electric fieldprobe to a pipe defect electrically connecting the metallic structuralcomponent to seawater.

The method may further comprise: determining the location of theelectric field probe relative to the flexible pipe: and determining thelocation of the pipe defect from the measured electric field vector andthe position of the electric field probe.

The method may further comprise: moving the electric field proberelative to the flexible pipe; and triangulating the location of thepipe defect from electric field vectors measured at two or morepositions of the electric field probe.

Advantageously, the detection apparatus allows a pipe body defect suchas a breach in an outer seawater resistant layer to be detected andlocated for an existing flexible pipe installation. That is, thedetection apparatus may be used once a defect such as a breach issuspected, and so the detection apparatus may be referred to as a postevent breach location apparatus.

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 illustrates a flexible pipe body;

FIG. 2 illustrates a riser assembly incorporating a flexible pipe body;

FIG. 3 illustrates a detection apparatus in accordance with anembodiment of the invention;

FIG. 4 is a graph illustrating the attenuation of an electrical signalrelative to the frequency of the signal for three different distances;

FIG. 5 illustrates an impedance monitor forming part of the detectionapparatus of FIG. 5;

FIG. 6 is a flow chart illustrating a detection method in accordancewith the embodiment of FIG. 2;

FIGS. 7 to 9 illustrates possible deployment scenarios for a detectionapparatus in accordance with certain embodiments of the presentinvention;

FIG. 10 illustrates a detection apparatus sensor suitable for mountingupon an ROV in accordance with an embodiment of the present invention;

FIG. 11 is an electrical schematic for a detection apparatus inaccordance with an embodiment of the present invention;

FIG. 12 is a circuit diagram for a detection apparatus in accordancewith an embodiment of the present invention;

FIG. 13 is a modelled electric field diagram illustrating the effect ofa seawater riser with a mid-way breach upon the electric field whenoperating a detection apparatus in accordance with an embodiment of thepresent invention;

FIG. 14 illustrates a detection apparatus in accordance with anotherembodiment of the invention; and

FIG. 15 is a flow chart illustrating a detection method in accordancewith the embodiment of FIG. 14.

In the drawings like reference numerals refer to like parts.

Throughout this description, reference will be made to a flexible pipe.It will be understood that a flexible pipe is an assembly of a portionof a pipe body and one or more end fittings in each of which arespective end of the pipe body is terminated. FIG. 1 illustrates howpipe body 100 is formed in accordance with an embodiment of the presentinvention from a combination of layered materials that form apressure-containing conduit. Although a number of particular layers areillustrated in FIG. 1, it is to be understood that the present inventionis broadly applicable to coaxial pipe body structures including two ormore layers manufactured from a variety of possible materials. It is tobe further noted that the layer thicknesses are shown for illustrativepurposes only.

As illustrated in FIG. 1, a pipe body includes an optional innermostcarcass layer 101. The carcass provides an interlocked construction thatcan be used as the innermost layer to prevent, totally or partially,collapse of an internal pressure sheath 102 due to pipe decompression,external pressure, and tensile armour pressure and mechanical crushingloads. It will be appreciated that certain embodiments of the presentinvention are applicable to ‘smooth bore’ operations (i.e. without acarcass) as well as such ‘rough bore’ applications (with a carcass).

The internal pressure sheath 102 acts as a fluid retaining layer andcomprises a polymer layer that ensures internal fluid integrity. It isto be understood that this layer may itself comprise a number ofsub-layers. It will be appreciated that when the optional carcass layeris utilised the internal pressure sheath is often referred to by thoseskilled in the art as a barrier layer. In operation without such acarcass (so-called smooth bore operation) the internal pressure sheathmay be referred to as a liner.

An optional pressure armour layer 103 is a structural layer with a layangle close to 90° that increases the resistance of the flexible pipe tointernal and external pressure and mechanical crushing loads. The layeralso structurally supports the internal pressure sheath, and typicallyconsists of an interlocked construction.

The flexible pipe body also includes an optional first tensile armourlayer 105 and optional second tensile armour layer 106. Each tensilearmour layer is a structural layer with a lay angle typically between10° and 55°. Each layer is used to sustain tensile loads and internalpressure. The tensile armour layers are often counter-wound in pairs.

The flexible pipe body shown also includes optional layers of tape 104which help contain underlying layers and to some extent prevent abrasionbetween adjacent layers.

The flexible pipe body also typically includes optional layers ofinsulation 107 and an outer sheath 108, which comprises a polymer layerused to protect the pipe against penetration of seawater and otherexternal environments, corrosion, abrasion and mechanical damage.

Each flexible pipe comprises at least one portion, sometimes referred toas a segment or section of pipe body 100 together with an end fittinglocated at one end or both ends of the flexible pipe. An end fittingprovides a mechanical device which forms the transition between theflexible pipe body and a connector. The different pipe layers as shown,for example, in FIG. 1 are terminated in the end fitting in such a wayas to transfer the load between the flexible pipe and the connector.

FIG. 2 illustrates a riser assembly 200 suitable for transportingproduction fluid such as oil and/or gas and/or water from a sub-sealocation 201 to a floating facility 202. For example, in FIG. 2 thesub-sea location 201 includes a sub-sea flow line 205. The flexible flowline 205 comprises a flexible pipe, wholly or in part, resting on thesea floor 204 or buried below the sea floor and used in a staticapplication. The floating facility may be provided by a platform and/orbuoy or, as illustrated in FIG. 2, a ship. The riser assembly 200 isprovided as a flexible riser, that is to say a flexible pipe 203connecting the ship to the sea floor installation. The flexible pipe maybe in segments of flexible pipe body with connecting end fittings. FIG.2 also illustrates how portions of flexible pipe can be utilised as aflow line 205 or jumper 206. It will be appreciated that there aredifferent types of riser, as is well-known by those skilled in the art.Embodiments of the present invention may be used with any type of riser,such as a freely suspended (free, catenary riser), a riser restrained tosome extent (buoys, chains), totally restrained riser or enclosed in atube (I or J tubes).

As noted above, defects in a flexible pipe body can compromise thestructural integrity of the pipe body. In particular, a breach orrupture of an outer seawater resistant layer can allow seawater ingressinto the pipe body annulus between an innermost barrier layer and theouter seawater resistant layer. With reference to FIG. 1 the outerseawater resistant layer may comprise the polymer outer sheath 108 andthe innermost barrier layer may comprise the internal pressure sheath102. The pipe body annulus is occupied by metallic structural componentssuch as the tensile armour layers 105, 106 of FIG. 1. Such componentsare frequently formed from steel or other metals and are susceptible torapid corrosion in the presence of seawater. There will now be describeddetection apparatuses and methods which can detect a breach of an outerresistant layer of a flexible pipe body.

FIG. 3 illustrates a detection apparatus coupled to a flexible pipe bodyin accordance with an embodiment of the present invention. The detectionapparatus is arranged to detect a change to a flexible pipe body whichmay indicate a defect (and in particular a breach allowing seawater orother fluids into the pipe body annulus). The detection apparatus may becoupled to a warning system arranged to provide an output signal to anoperator of the flexible pipe alerting the operator to potential damageto the pipe. The output signal may, for instance, be a visual or audiblealarm.

As discussed above, a flexible pipe body may be constructed from atleast one layer of polymer barrier, including an outer seawaterresistant layer 304 and at least one layer of co-axial metallicstructural elements 306, 308. The metallic structural elements, forinstance the tensile armour layers 105, 106 of FIG. 1, are designed tosatisfy purely mechanical properties of the structure of the pipe body.However, they are electrically conductive and this property is exploitedin accordance with certain embodiments of the present invention.Detection apparatuses in accordance with certain embodiments of theinvention are arranged to be used to detect the location of a breachwithin a flexible pipe with at least one metallic layer or structuralcomponent. As will be described in greater detail below, the detectionapparatus is arranged so that it can be used to detect the location of abreach of an existing flexible pipe installation and so no modificationof the pipe body is required.

It will be appreciated that in the event of a breach of an outerseawater resistant layer the pipe body annulus will begin to fill withseawater. Metallic structural components within the pipe body annulusthus come into electrical contact with the seawater surrounding the pipebody, and so a change in impedance between the seawater surrounding thepipe body and the metallic structure of the pipe body will result. Adetection apparatus in accordance with certain embodiments of theinvention is arranged to measure the impedance between the metallicstructural components of a pipe body and a seawater electrode, fromwhich it can be deduced whether there is a breach and the approximatelocation of the breach. The skilled person will understand thatmeasuring the impedance between these two points is directly equivalentto measuring the conductivity of the seawater between these two points.

As will be described below, the detection apparatus illustrated in FIG.3 couples to an electrically conductive structural component extendingat least partially along the length of a pipe body. Metallic structuralcomponents with flexible pipes are typically electrically coupledtogether at an end fitting and the end fitting in turn is coupled to thelocal Earth upon the production platform to reduce the risk ofexplosion. The detection apparatus is coupled to the local Earth uponthe production platform, and thereby indirectly coupled to the metallicstructural components of the pipe body. The electrically conductivemember may comprise a metallic structural component such as a singletensile armour wire or a tensile armour wire layer.

The outer seawater resistant layer of a flexible pipe body may bemanufactured from a polymer material with known, intrinsic electricalinsulation properties. Seawater has known electrical conductionproperties, though this may vary from location to location, for instancedue to variation in the salinity of the seawater, and so a detectionapparatus in accordance with an embodiment of the invention may requirecalibration before use to adapt to local conditions. A physical breachin the form of an aperture in the outer seawater resistant layer of aflexible pipe permits a conductive path between the seawater and thesteel internal structure of the pipe body. In accordance with certainembodiments of the present invention an electrical impedance measurementmade between seawater surrounding a flexible pipe and the internalmetallic structure of the pipe body provides a means of indicating thepresence of a breach. Specifically, in the event that the measuredimpedance is below a certain threshold, it can be inferred that a breachhas occurred and seawater is in contact with the internal metallicstructure of the pipe body.

Referring in detail to FIG. 3, this shows a flexible pipe body 400,which as discussed above may comprise a riser. The pipe body is at leastpartially surrounded by seawater, schematically illustrated by the pipebody 400 extending below the surface level 402 of the sea. As discussedabove, such a flexible pipe body is constructed from multiple layers ofpolymer barrier, including an outer seawater resistant layer 404 and atleast one layer of metallic structural elements 406, for instance thetensile armour layers 105, 106 of FIG. 1. A seawater electrode 408 is incontact with seawater in proximity to the pipe body 400. As will bedescribed in greater detail below in connection with FIGS. 7 to 10, thelocation of the seawater electrode 408 may be varied in order toascertain the location of a breach to a greater accuracy, oralternatively multiple seawater electrodes 408 may be provided to allowthe triangulation of the location of a breach. Each seawater electrode408 may alternatively be referred to as a probe or a conductivity probe.

An impedance monitor 410 is coupled to the seawater electrode 408 and toa metallic structural component 406 of the pipe body 400 via the localEarth upon the production platform. The impedance monitor 410 provides ameasurement of the impedance between the seawater electrode 408 and thepipe body 400 (alternatively, this can be considered to be aconductivity measurement). The impedance measurement is supplied to aprocessor 412 for analysis. If the measured impedance is less than athreshold when the seawater electrode is proximate to the pipe body thenthis may indicate that there is a breach. For instance, a very highimpedance measurement could indicate that there is no breach. Theimpedance of a polymer barrier layer is approximately 1 MΩ. However, theapproximate impedance of seawater is 5Ω and so a low measured impedancemay indicate that seawater has penetrated the pipe body annulus througha breach. As discussed in greater detail below care must be taken toensure that the measured impedance is not that of the seawater betweenthe seawater electrode and the hull of the production vessel whichtypically forms the local Earth. In one embodiment the impedance monitor412 is arranged to measure impedance in the range of 0-10 kΩ. Animpedance above 10 kΩ is registered as the maximum 10 kΩ due to themeasurement system saturating at that value. Advantageously, thisaccurately records the absence of a polymer barrier layer while allowinggreater measurement resolution at lower impedance values. If there isany seawater conductivity path between the structure of the pipe bodyand the seawater electrode (when the seawater electrode is relativelyclose to the location of a breach) the measured impedance is well below10 kΩ. The processor 412 is arranged to provide an appropriate outputsignal to an operator of the flexible pipe alerting the operator topotential damage to the pipe.

Unlike the predominantly electron flow conduction in metals, theelectrical conduction in seawater is dependent on ion mobility, and thisleads to significant variation in observed conductivity with thefrequency of the applied measurement excitation. This is shownschematically in the FIG. 4 which shows relative attenuation of anapplied alternating current signal at various low frequencies andbetween electrodes spaced at 10 m, 100 m, and 1 Km. Certain embodimentsof the present invention takes advantage of the attenuation data of FIG.4 by applying frequency agile excitation of the impedance monitor 410.Measurement of the impedance at a single excitation frequency can givean indication of the distance through the seawater between the seawaterelectrode 408 and the breach. Certain embodiments of the invention takeadvantage of this by moving the seawater electrode between differentlocations relative to the pipe body to increase the measurement accuracyby triangulating the results. However, as noted above, due to thevariation in seawater conduction, additionally or alternatively theimpedance monitor may be excited at two or more frequencies allowing theresults to be compared, and from that information an approximatelocation of the breach determined. In certain embodiments the excitationfrequency of the impedance monitor is in the range 10 Hz to 1 kHz. Inother embodiments the maximum excitation frequency of the impedancemonitor may be 100 kHz. It will be appreciated by the skilled personthat the detection system of FIG. 3 could also operate using DC testsignals to determine whether there is a pipe defect through detectedseawater conduction to the seawater electrode. However, it will beunderstood that this would not allow the detection of the location ofthe defect.

One embodiment of the impedance monitor is shown in greater detail inFIG. 5. A first electrode 500 is coupled to the local Earth upon theproduction platform (and hence connected to the metallic structuralcomponents of the pipe body) and a second electrode 502 is coupled tothe seawater electrode. The second electrode 502 is coupled between aHowland current source 504 and a synchronous demodulator 506. Theimpedance monitor may operate in either a voltage or current sourcemode. In FIG. 5 a Howland current source 504 is used as it provides agood linearity of response. The Howland current source 504 is shownconnected to the second electrode. The Howland current source 504 thussupplies a current to the seawater electrode in response to an inputsignal supplied by a synchronous filter 508. The seawater electrode thusenergises the seawater surrounding the pipe body generating an electricfield extending between the seawater electrode and the productionplatform and any breach within the pipe body outer seawater resistantlayer (as will be explained in greater detail below in connection withFIG. 13). The current may be AC current. In one embodiment, preferablythe signal may be a sinusoidal waveform AC current. Other waveforms, forinstance a square wave could be used, however sinusoidal is preferredbecause then there are no harmonics present, which could interfere withthe operation of the frequency dependent range measurement system. Thatis, the applied electrical test signal may be AC. In other embodiments avoltage source may be used. The synchronous filter 508 provides a signalunder the control of a pulsed control signal from controller 510, whichadditionally supplies the same control signal to the synchronousdemodulator 506. The synchronous demodulator 506 is arranged to analysethe voltage generated across the seawater between the seawater electrodeand the Earth of the production vessel at each frequency. Thesynchronous demodulator 506 supplies an output signal to the controller510, which is indicative of the voltage of the second electrode 502relative to Earth. In the event of a breach of the polymer barrier, thevoltage of the second electrode 502 is dependent upon the appliedcurrent and the seawater impedance between the electrodes 500, 502indicated by resistor symbol 512. This may only be the case if theseawater electrode is positioned relatively close to the location of thebreach, and so certain embodiments of the present invention allow forthe location of the seawater electrode to be varied in order to ensurethat the conduction between the seawater electrode and the breach is notswamped by the conduction between the seawater electrode and theproduction platform. The controller 510 is arranged to generate anoutput signal indicative of the impedance between the electrodes 500,502 by comparison of the supplied current and the measured voltage. Theoutput signal is provided to the processor 412, which is arranged todetermine whether a breach is detected.

In accordance with certain embodiments of the present invention theprocessor 412 is arranged to instruct the impedance monitor to performimpedance measurements at a range of excitation frequencies (forinstance 10 Hz, 30 Hz, 100 Hz, 300 Hz and 1 kHz), which by crossreference to the graph of FIG. 4 (or by reference to a look up tablewithin the processor 412) allow an estimation of the position of abreach to be determined. The accuracy of this computation is dependenton a number of factors including the size of the breach, the salinityand temperature of the seawater, the attitude of the pipe body (e.g.vertical to horizontal) and the electrical conductivity of the steelinner structure.

Referring now to the flow chart of FIG. 6, a detection method inaccordance with the present invention will now be described. At step 700the detection apparatus illustrated in FIG. 3 is coupled to anelectrically conductive member within the pipe body (via the Earth uponthe production platform) and to a seawater electrode. At step 702 theimpedance between the electrodes is measured for at least one firstfrequency. At step 704 the frequency and impedance data is used todetermine the location of a breach.

Advantageously, embodiments of the present invention described above donot interfere with active cathodic protection systems coupled to pipebodies owing to the fact that no excitation signal is applied to themetallic structural components of the pipe body.

In accordance with certain embodiments of the invention the seawaterelectrode (also referred to as a seawater conductivity probe) isenergised with at least one frequency or a low swept or switchedfrequency to measure the return Earth path through a breach to themetallic structural components of the pipe body (which in turn areconnected to Earth). In order to accurately locate a breach requiresknowledge of the location of the probe relative to the pipe body and theinferred distance from the seawater electrode to the breach (asdescribed above). Advantageously, if the distance to the breach can bedetermined from multiple locations then triangulation may be used toprovide a more accurate determination of the breach location.Measurements from multiple locations can be achieved either by movingthe probe or by providing multiple probes. The seawater conductivityprobe may use a swept low-frequency signal. Preferably, a series ofreadings are taken around the riser along its length. Triangulationindicates a breach location. Conductivity data is typically transmittedback to the surface and collated for analysis. Conductivity measurements(including the frequency used and signal strength) can be overlaid on adata indicating the probe location to build a 3D model of conductivity.When overlaid on a 3D model of the subsea pipes a breach location can beidentified.

Referring now to FIG. 7 this illustrates two possible deploymentapproaches for a probe. FIG. 7 shows a production platform 700 (avessel) and a flexible pipe 702 extending downwards from the productionvessel 700 and surrounded by seawater. The first approach is to dip theprobe 704 over the side of the production platform attached to a cable706. This probe reads the seawater conductance from the probe to thebreach 708 marked R. This drawing is a simplification of the principleas the vessels hull acts as an additional (and large) return path toEarth. Depending upon the location of the breach 708 this backgroundpath may render it impossible to locate the breach. The basic probeapproach is capable of detecting the breach location to approximately100 m or better depending upon how close the cable 706 runs relative tothe pipe 702. The depth of the probe may be indicated by the length ofcable paid out Conductivity measurements may be performed at multipledepths, with the depth having the largest conductance measured beinglikely to indicate the depth of the breach along the length of the pipe702. Used like this without steering of the probe, the detectionapparatus may be used for initial survey work. The cable 706 ispreferably combined with communications and power supply cables forcontrolling the probe. The probe 704 may be dipped over the side of thevessel 700 at multiple locations around the pipe 702 to increase themeasurement accuracy.

Once the dipping probe has given an approximate breach location (or inplace of performing dipping measurements) a probe coupled to a RemotelyOperated Vehicle 710 (ROV) may be used to steer the location of theprobe close to the location of a suspected breach. As an alternative tousing an ROV, any other means for actively steering the position of theprobe may be used. The ROV 710 may be coupled to the production vessel700 by an umbilical 712, which also serves to supply power andcommunications to the probe as for the dipping probe 704. The ROV'sposition is tracked using a 3D sonar position system which can be usedto map out the background return paths to allow a better prediction ofthe breach location. Implementation also requires a sub-sea chart ofdeployed production equipment.

The ROV needs to be unearthed while in the water to not generate its ownreturn path. Operating a probe coupled to an ROV may allow the breachlocation to be detected to an accuracy of approximately 20 m or better.

Referring now to FIG. 8, in accordance with a further embodiment anarray of probes may be provided. For instance, the array may be providedin the form of a towed array cable 800 which has multiple individualconductors 802 positioned axially along a cable. The deployment of thetowed array cable 800 is generally the same as for the dipping probe 704and cable 706 shown in FIG. 7. The towed array cable 800 can bepositioned next to the pipe 702 and each conductor 802 can bemultiplexed to read the conductivity at each point along the cable 800.Advantageously, this embodiment allows the measurement of the distanceto the breach from multiple locations (multiple depths below the surfaceof the sea) without requiring that the cable is moved. This allows forthe rapid collection of data to aid possible ROV deployment.

Referring now to FIG. 9, in accordance with a further embodiment one ormore conductive hoops 900 may be placed around the outside of the pipe702. Each hoop may comprise either a single conducting element ormultiple conducting elements spaced around the circumference of the pipe702. If multiple conducting elements are provided spaced apart aroundthe circumference of the pipe 702 then the orientation of a breacharound the circumference of the pipe 702 may be established. Eachconductive hoop 900 is coupled to a cable 902 running up to the vessel700 for communications and power, as for the embodiments of FIGS. 7 and8. If a single hoop 900 is used the hoop 900 can be lowered and readingstaken to locate the breach. If a single conducting element is used, thismethod gives a 360 degree return path and due to its locality with abreach an accurate location is provided. A hoop array 904 formed frommultiple hoops 900 may be deployed by sliding along the pipe 702.Alternatively, a hoop array 904 may be preinstalled by positioning thehoops along the pipe 702 or incorporating the hoops 900 into the pipe orcomponents such as joints and buoyancy.

Referring now to FIG. 10, in accordance with a further embodiment, aclamp 1000 may be attached to an ROV 1002 and arranged to be coupledaround a pipe 1004 (shown in cross section). For instance, the clamp1000 may include a hinged portion (not shown) arranged to open to admitthe pipe 1004. The pipe 1004 shows a metallic structural layer 1006 andan outer seawater resistant layer 1008 in which a breach 1010 is formed.The clamp 1000 includes multiple conductors 1012 which may be separatelyenergised so that the orientation of the breach about the circumferenceof the pipe 1004 may be ascertained. A clamp 1000 coupled to an ROV 1002may thus be used to locate and map a breach both along the risers lengthand circumference. The conductors 1012 may initially be connectedtogether to form a ring to detect the breach axially along the pipe1004. Once the breach area is detected the conductors 1012 may beindividually scanned to locate the breach position on the circumferenceof the pipe 1004. This method could also be used to measure the extentof a breach, for instance the circumferential size and axial length of aslit in the outer seawater resistant layer 1008. This method greatlyreduces the vessels Earth return background interference due to theprobe being placed in close proximity to the pipe.

As noted above, owing to the fact that the conductivity of seawater isvariable, for a particular deployment location it may be desirable tocalibrate the detection apparatus. For instance, this calibration may beachieved by attaching a metal plate to a pipe. The metal plate isconnected to the Earth of the production vessel via an insulated wire,such that the metal plate simulates a breach at a known location.

As noted above, existing pipe installations do not readily allow forenergising the metalwork within the pipe in a manner to create a signalthat can be remotely detected. This is due to the fact that metalliccomponents are electrically connected together through the pipeend-fitting, which is itself bonded to the zero volts potential (Earth)on board the production platform. As discussed above, the presentinvention instead relies upon a seawater conductivity probe to energisethe seawater surrounding the pipe, which completes a circuit through abreach in the riser coating to the metal work of the pipe body.Referring now to FIG. 11 this is an electrical schematic illustratingthe measured conductivity path. Metallic structural components 1100within the pipe 1102 are exposed to the surrounding seawater through abreach 1104 in the outer seawater resistant layer of the pipe body. Themetallic structural components 1100 are electrically coupled to a topside end fitting (not shown) and then on to the zero volts potential(Earth) 1106 on board the production platform 1108. A seawaterconductivity probe 1110 is positioned proximate to the pipe 1102 (via acable or ROV as discussed above), and the probe 1110 is coupled by wire1112 to Earth 1106. The probe 1110 is energised, for instance using theimpedance monitor of FIG. 5 (not shown) in order to provide a measure ofthe conductance (or, equivalently, the impedance) of the seawater path1114 between the probe 1110 and the breach 1104. FIG. 11 further shows acommunications cable 1116 extending between the probe 1110 and a PC 1118(or equivalent data analysis and collation equipment) on board theproduction platform 1108. While data collation could be performed withinthe probe 1110 for later analysis or real time analysis at the probe1110, it is preferable that data is returned to the relative security ofthe production platform 1108.

Referring now to FIG. 12, this illustrates a simplified electricalcircuit of a detection apparatus 1200 in use in accordance with anembodiment of the present invention. A low frequency constant currentsource 1202 may be used to determine the total circuit conductivity, onthe assumption that the internal resistance 1204 of the pipe body andthe production platform remains unchanged. In practice, the internalresistance 1204 may vary according to how far along the pipe the breachis, though this internal resistance is relatively small compared to theseawater resistance 1206 in the event of a breach and so may be measuredor modelled before a breach occurs and assumed to remain constant. Thecurrent source 1202 is used to energise the circuit such that theseawater resistance 1206 can be measured with reference to the voltageacross a standard resistor 1208 using a voltmeter 1210.

As discussed above, given that the resistance of seawater varies withfrequency (as shown in FIG. 4) driving the constant current source 1202with different frequencies allows the system to effectively providedifferent ranges of operation. This may advantageously be used byoperating at a relatively low frequency when the probe is remote fromthe pipe. Alternatively, higher frequency ranges allow discrimination ofbreach locations over shorter distances, and so allows very largealternative current paths that are not of interest (in particularthrough the hull of the production platform) to be filtered out toreveal the small localised return path through the breach. The need todiscriminate return paths can be dearly seen by reference to FIG. 13which shows a modelled electrical field (indicated by equipotentialfield lines 1300. FIG. 13 shows a model of a production platform hull1302, a breach 1304 part way along a pipe (not shown) and the pipebottom 1306 at which point the metallic structure of the pipe body istypically again exposed to the seawater by connection to an aluminiumanode for cathodic protection. The hull 1302, breach 1304 and pipebottom 1306 are all at the same potential (the zero volts, Earth, of theproduction platform). A probe 1308 is shown relatively close to thebreach 1304 and field lines 1310 are shown coupling the probe 1310 toeach of the hull 1302, breach 1304 and pipe bottom 1306. It can be seenthat the hull 1302 forms a significant return path and so has asignificant effect on the apparent conductivity of the seawater. Toavoid this, for possible breach locations close to the ship, higherfrequencies may be used with the probe close to the pipe (for instance,less than 10 m). Further down the pipe, a lower frequency can be usedfurther from the pipe body (for instance, 100 m) to allow for more rapidscanning for breaches. The effect of the ship's hull and the bottomend-fitting is to create dead zones where it may be difficult orimpossible to locate a breach, even with the use of higher frequencies.This effect will be largest for the ship's hull due to its sheer size.

As discussed above, analysis using two different frequencies isdesirable. By operating the probe at several distinct alternatingfrequencies with known differing conductivities in seawater the locationof a breach may be more accurately determined. However, it will beappreciated that embodiments of the present invention may locate abreach using only a single frequency, with increased accuracy achievedby moving the probe closer to the suspected location of the breachand/or performing triangulation using several measurements taken fromdifferent locations. Phase sensitive conductivity/impedance measurementallows for more sensitive detection.

In accordance with an alternative embodiment of the present invention,in place of measuring the impedance between a mobile probe and thebreach as described above, an electric field may be established betweena static probe and the breach. A second mobile probe may then measurethe electric field surrounding the pipe in all three planes at knownpositions. This way, the electric field pattern resulting from thestatic probe energising the seawater can be examined for both magnitudeand direction. An electric field vector measured relatively close to thelocation of a breach may indicate the location of the breach. Phasesensitive detection allows the polarity of the detected field to bedetermined, such that the vector may directly point to the location ofthe breach.

Referring to FIG. 14, this illustrates a detection apparatus inaccordance with this alternative embodiment of the invention. Pipe 1400is shown extending underneath the surface of the sea 1402 and pipe 1400has a breach 1404. A static seawater electrode 1406 is energised bysignal generator 1408 which is also coupled to the metallic structure1410 of the pipe body via the local Earth of the production platform(not shown). The static electrode 1406 is energised using a constantvoltage source to establish an electric field in the seawatersurrounding the pipe 1400. This differs from the constant current sourcedescribed in previous embodiments used to perform conductivity/impedancemeasurements. A signal analyser 1412 is coupled to the signal generatorsuch that the measured electric field can be correlated to the phase ofthe signal supplied to the static electrode 1406 allowing the directionof an electric field vector to be established. A second probe 1414 isprovided that can be moved relative to the pipe 1400, for instance bybeing attached to a cable and dipped into the sea or by being coupled toan ROV, as described above. The mobile probe 1414 is coupled to thesignal analyser to provide an indication of the electric field measuredin three dimensions. The location of the mobile probe may be directlydetermined using a 3D sonar transponder as described above.

This electric field vector approach offers considerable advantages overthe above described techniques. Not only does it have a greatersensitivity, but also the vector allows the breach to be physicallypointed to. This means that when deployed in the field, fewermeasurement points need to be recorded. The mobile probe 1414 is mobilewithin a zone in the vicinity of the pipe 1400, but distant from theenergising electrode 1406 to ensure that the measured vector does notsimply point to the electrode 1406. The mobile probe 1414 may comprisethree pairs of opposing electrodes spaced a small distance apart (forinstance 0.5 m) apart, each oriented at right angles to the other tomeasure the electric field in three parallel planes. Three differentialamplifiers within the sensor establish the electrode potential betweeneach individual pair of electrodes, and from this information, themagnitude and direction (that is, the vector) of the electric fieldwithin the seawater at the point of measurement can be established. Bymoving the probe's position within the zone, and noting the vectorvalues, the electric field vector pattern may be analysed to indicatethe position of the breach with a degree of accuracy that typicallysurpasses that of the previously described embodiments. Interpretationof the results of the electric field measurement require knowledge ofthe position of the mobile probe, for instance using a sonar transponderas described above, and also the orientation of the probe, which may beestablished using gyroscopes, for instance MEMS based gyroscopes. Anumbilical connecting the probe to the surface carries, as well as power,a reference signal at the delivered frequency, as this is required toallow the sign of the vector to be established.

Referring now to FIG. 15 this is a flowchart illustrating a method ofoperating the detection apparatus of FIG. 14. At step 1500 the staticelectrode is deployed and used to establish a static electric field inthe seawater surrounding a potential breach. At step 1502 the electricfield is measured at a first point using the mobile probe in order tolocate the breach. Depending upon the electric field vector measured atstep 1502, the mobile probe is moved towards the suspected breachlocation at step 1504. This may be an iterative process in which themobile probe is move progressively towards a potential breach until itsprobable location is determined to a sufficient degree of accuracy.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean “including but notlimited to”, and they are not intended to (and do not) exclude othermoieties, additives, components, integers or steps. Throughout thedescription and claims of this specification, the singular encompassesthe plural unless the context otherwise requires. In particular, wherethe indefinite article is used, the specification is to be understood ascontemplating plurality as well as singularity, unless the contextrequires otherwise.

Features, integers or characteristics described in conjunction with aparticular aspect, embodiment or example of the invention are to beunderstood to be applicable to any other aspect, embodiment or exampledescribed herein unless incompatible therewith. All of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), and/or all of the steps of any method or processso disclosed, may be combined in any combination, except combinationswhere at least some of such features and/or steps are mutuallyexclusive. The invention is not restricted to the details of anyforegoing embodiments. The invention extends to any novel one, or anynovel combination, of the features disclosed in this specification(including any accompanying claims, abstract and drawings), or to anynovel one, or any novel combination, of the steps of any method orprocess so disclosed.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

We claim:
 1. A detection apparatus arranged to detect defects within aflexible pipe at least partially surrounded by seawater, the detectionapparatus comprising: a seawater electrode arranged to be in contactwith seawater surrounding at least part of a flexible pipe; an impedancemonitor arranged to measure the impedance between a metallic structuralcomponent of the flexible pipe extending at least partially along thelength of the flexible pipe and the seawater electrode in response to anelectrical test signal applied to the seawater electrode; and aprocessor arranged to determine the distance from the seawater electrodeto a pipe defect electrically connecting the metallic structuralcomponent to seawater using the measured impedance.
 2. A detectionapparatus according to claim 1, wherein the seawater electrode isarranged to move relative to the flexible pipe, the impedance monitor isarranged to measure the impedance between the metallic structuralcomponent and the seawater electrode at two or more positions and theprocessor is arranged to determine the distance from the seawaterelectrode to the pipe defect for each position and to triangulate thelocation of the pipe defect.
 3. A detection apparatus according to claim2, wherein the seawater electrode is arranged to be lowered through theseawater surrounding the flexible pipe or the seawater electrode iscoupled to a steering mechanism such that the location of the seawaterelectrode relative to the flexible pipe can be controlled.
 4. Adetection apparatus according to claim 1, further comprising two or morespaced apart seawater electrodes separately coupled to the impedancemonitor; wherein the impedance monitor is arranged to measure theimpedance between the metallic structural component and each seawaterelectrode and the processor is arranged to determine the distance fromeach seawater electrode to the pipe defect and to triangulate thelocation of the pipe defect.
 5. A detection apparatus according to claim1, further comprising a position locator coupled to the seawaterelectrode and arranged to provide an indication of the location of theseawater electrode relative to the flexible pipe; wherein the processoris arranged to determine the location of the pipe defect from themeasured impedance and the position of the seawater electrode.
 6. Adetection apparatus according to claim 1, wherein the seawater electrodecomprises a loop arranged to pass around the flexible pipe, the loopincorporating two or more conducting elements spaced around the flexiblepipe, the conducting elements being arranged to be coupled together andconnected to the impedance monitor or separately connected to theimpedance monitor.
 7. A detection apparatus according to claim 1,wherein the processor is further arranged to determine the location ofthe pipe defect using a three dimensional model of the location of theflexible pipe.
 8. A detection apparatus according to claim 1, whereinthe impedance monitor is arranged to measure the impedance for testsignal applied to the seawater electrode at a first frequency selectedaccording to the distance between the seawater electrode and theflexible pipe.
 9. A detection apparatus according to claim 1, whereinthe impedance monitor is arrange to measure the impedance for two ormore test signal frequencies applied to the seawater electrode, and theprocessor is arranged to determine the distance from the seawaterelectrode to a pipe defect by comparison of the measured impedances atthe first and second frequencies.
 10. A detection apparatus according toclaim 9 wherein the impedance monitor is arranged to apply electricaltest signals to the seawater electrode at a plurality of frequenciesbetween 10 Hz and 100 kHz.
 11. A method of detecting defects within aflexible pipe at least partially surrounded by seawater, the methodcomprising: immersing a seawater electrode into seawater surrounding atleast part of a flexible pipe; coupling an impedance monitor between ametallic structural component of the flexible pipe extending at leastpartially along the length of the flexible pipe and the seawaterelectrode; generating an electrical test signal and applying theelectrical test signal to the seawater electrode; measuring theimpedance between the metallic structural component and the seawaterelectrode in response to the test signal; and determining the distancefrom the seawater electrode to a pipe defect electrically connecting themetallic structural component to seawater using the measured impedance.12. A method according to claim 11, further comprising: moving theseawater electrode relative to the flexible pipe; measuring theimpedance between the metallic structural component and the seawaterelectrode at two or more positions; determining the distance from theseawater electrode to the pipe defect for each position; andtriangulating the location of the pipe defect.
 13. A method according toclaim 11, further comprising: immersing two or more spaced apartseawater electrodes into seawater surrounding at least part of theflexible pipe, each seawater electrode being separately coupled to theimpedance monitor; measuring the impedance between the metallicstructural component and each seawater electrode; determining thedistance from each seawater electrode to the pipe defect; andtriangulating the location of the pipe defect.